(1) Field of the Invention
The present invention is related to the field of treatment of electromagnetic energy, in particular of optical energy, more specifically in the infrared and visible wavelengths, and concerns a device for sorting and concentrating electromagnetic energy of radiations with different properties, as well as an apparatus comprising at least one such device.
(2) Description of Related Art
Even though the present invention is more specifically described in relation to visible light or infrared radiations, it should be understood that a much wider range or types of electromagnetic radiations can be impacted by this invention, more specifically radiations ranging in matter of wavelength from microwaves to ultraviolet radiations.
In particular, in a wide variety of optical sensing applications, there is a need for devices able to characterize the spectrum (i.e. wavelength distribution) of incoming radiation (light). There are also applications which require characterization of the polarization state of the radiation or light. Almost invariably in these applications, there is a need to make efficient use of the available light while preserving signal integrity. This is normally combined with constraints on device size and/or cost.
Current imaging technology is based on arrays of light sensors, for example arrays based on silicon CCD or CMOS technology. In a basic imaging system using a two-dimensional matrix array of light sensors, the signal from each sensor element corresponds to one pixel in the image.
Spectral and polarometric imaging faces a general technological challenge in that the image data are essentially three-dimensional (with spectral or polarimetric information constituting a third dimension) while light sensor arrays generally are limited to two dimensions.
For color imaging, it is common to use a matrix array of light sensors equipped with individual color filters, each passing one of the primary colors, arranged in a so-called “Bayer pattern” or similar.
In this case, the partial images corresponding to each primary colour are not spatially registered, and some form of interpolation is needed to estimate, for each pixel, the amount of light in the two primary colours not passed by the spectral filter of the corresponding light sensor element. This can lead to color artefacts in the resulting image.
Also, each light sensor element collects only light within its own spectral band, and thereby the rest of the light impinging on that particular element is lost.
There is a general demand to overcome the limitations exposed herein before, for applications in connection with the handling, treatment and/or exploitation of visible light, as well as other electromagnetic radiations having longer or shorter wavelengths.
On the other hand, there has been, in recent years, significant progress in the understanding and exploitation of charge oscillation phenomena at optical frequencies on metal surfaces, a field known as plasmonics [see for example: Barnes et al., Nature vol. 424, p. 824, 2003; C. Genet and T. W. Ebbesen, Nature vol. 445 p. 39 (2007)].
Indeed, surface plasmons (SPs) have generated considerable interest recently due to their potential in optics and sensing, among numerous other applications [see for example: Zayats, A. V., Smolyaninov, I. I & Maradudin, A. A. “Nano-optics of surface plasmon polaritons.” Phys. Rep. 408, 131-314 (2005) ; Mikhailov, V., Wurtz, G., Elliot, J., Bayvel, P., Zayats, A. V. “Dispersing Light with Surface Plasmon Polaritonic Crystals.” Phys. Rev. Lett. 99, 083901 (2007); Zia, R., Schuller, J. A., Chandran, A., Brongersma, M. L. “Plasmonics: the next chip-scale technology.” Materials Today. 9, 20-27 (2006)].
Surface plasmons are essentially light waves trapped at a metal surface by their interaction with the free electrons in the metal. Their properties can be controlled by texturing the metal surface. In the context of spectral and polarizing imaging, single apertures surrounded by periodic grooves are of particular interest [see for example: Thio, T., Pellerin, K. M., Linke, R. A., Lezec, H. J. & Ebbesen, T. W. “Enhanced light transmission through a single subwavelength aperture.” Opt. Lett. 26, 1972-1974 (2001); Nahata, A., Linke, R. A., Ishi, T. & Ohashi, K. “Enhanced nonlinear optical conversion from a periodically nanostructured metal film.” Opt. Lett. 28, 423-425 (2003); Garcia-Vidal, F. J., Lezec, H. J., Ebbesen, T. W. & Martin-Moreno, L. “Multiple paths to enhance optical transmission through a subwavelength slit.” Phys. Rev. Lett. 90, 213901 (2003)].
The periodic grooves act like an antenna for the incoming light by converting it to surface plasmons and enhancing the transmission through the aperture.
FIG. 1 gives an example of one such aperture structure where a single subwavelength hole is surrounded by concentric grooves.
Such a structure, also known as “bull's eye” structure can, for example, be milled by focused ion beam (FIB) lithography in the surface of a metal film (for example made of silver or gold). Such a device may be for visible light for example a 300 nm thick Ag film on a glass substrate with a structure having the following dimensions: diameter of central hole: 170 nm; width of grooves: 150 nm; depths of grooves: between 10 nm and 150 nm; spatial period: 600 nm.
As indicated by the arrows, incident light excites surface charge oscillations, and the optical energy is concentrated at the center of the structure where it enters an aperture in the film. The light emerges at the back side of the film where, depending on the structuring of the back side, the light may diffract out as indicated. The structure preferentially collects light within a band of wavelengths determined mainly by the period of the ring structure. At the bottom is an image of an actual bull's eye structure fabricated in a gold film.
The transmission peak wavelength λSPP of such a structure can be tuned by controlling the groove periodicity P (FIG. 1b) as predicted by equation (1) for normal incidence illumination:
                              λ          SPP                =                  P          ⁢                                                                      ɛ                  m                                ⁢                                  ɛ                  d                                                                              ɛ                  m                                +                                  ɛ                  d                                                                                        (        1        )            
where ∈m and ∈d are the dielectric constants of the metal and the dielectric in immediate contact with the metal surface. The peak is normally red-shifted compared to the prediction of equation (1) due to Fano-type interaction. The transmission spectrum is also modulated by the other structural parameters such as groove depth, width, aperture shape and size, as indicated in the previously quoted references.
Such a structure preferentially collects optical energy within a band of wavelengths adapted to its period.
The surface plasmons give rise to intense electromagnetic fields at the central hole. The flux per unit area through the aperture can be larger than that of the incident light, confirming that the grooves act as an antenna, collecting light laterally from an area around the hole. This extraordinary transmission phenomenon allows for useful light collection efficiencies even though the apertures represent only a small fraction of the surface of the device, the optical energy being extracted at the back of the film, at the exit opening of the aperture, after passing through the latter.
Another example of a known radiation or light concentrating structure consists of a metal film with linear corrugations surrounding a slit-shaped aperture in which the optical or radiative energy is concentrated, as illustrated in FIG. 2. The concentrating effect of this structure depends not only on the wavelength, but also on the polarization of the incoming light or radiation.
Now, the inventors have discovered unexpectedly that plasmonics based surface collector structures can be applied to realise at least a spectrally differentiated or polarisation selective collection, and therefore a characterisation of the spectral properties, of electromagnetic radiations, and in particular of light, which allows in particular to overcome the above mentioned limitations in respect of the known Bayer-mask technique.